A novel process has been discovered for preparing a polyorganosiloxane/polyvinyl-based rubber substrate in emulsion. Subsequent graft polymerization results in a material which is extremely useful as an impact modifier for various thermoplastic resins. Many benefits can be realized from utilizing this new process over other conventional processes which do not contain the polyorganosiloxane/polyvinyl rubber substrate. For example, a highly cross-linked rubber substrate can be obtained by the present process. Improved subsequent stage grafting efficiency and lower manufacturing costs are additional benefits
There have been many attempts in the art to provide polyorganosiloxane-based graft polymers which may be useful as impact strength modifiers for thermoplastic resins. See for example, U.S. Pat. No. 2,891,920 (J. F. Hyde, et al.); and O. Graiver, et al., Rubber Chem. Tech., 56 (5), 918 (1983).
The major deficiencies which have prevented the widespread use of polyorganosiloxane impact modifiers in thermoplastic resins have included raw material costs, relatively poor rubber integrity, and the incompatibility between the silicone-based rubber modifier and the thermoplastic resin. Additionally, the siloxane polymerization process requires careful control to eliminate contamination of the silicone rubber by linear or cyclic siloxane oligomers. Surface delamination in molded thermoplastic parts has been partially attributed to the presence of such oligomer contaminants in the silicone rubber.
Furthermore, polyorganosiloxane compounds generally exhibit low reactivity toward vinyl monomers during the course of subsequent graft polymerization. Several methods for improving graft efficiency have been reported. U.S. Pat. No. 3,898,300 states that a polyorganosiloxane-based graft copolymer for improving the impact strength of S/AN resin is formed by grafting S/AN comonomers in an emulsion system onto the vinylsiloxane or allylsiloxane containing silicone substrate. U.S. Pat. No. 4,071,577 describes a similar approach by using a mercaptosiloxane in place of vinyl-group containing siloxanes. European Patent Application No. 0166900 reports further improvement of polysiloxane-based graft polymers and increased S/AN impact strength by using an acryloxy-functionalized siloxane as the graftlinking agent. These graft polymers are utilized in connection with the impact modification of S/AN. British Patent No. 1,590,549 describes the use of a polyorganosiloxane graft polymer in various plastic molding compositions. Similarly, European Patent Application 0249964 describes the use of a polyorganosiloxane graft polymer in polycarbonate containing blends.
Mention is also made of European Patent Application Nos. 0246537 and 0260552, both of which describe the use as impact modifiers of a polyorganosiloxane polymer substrate on which are subsequently grafted first and second vinyl-based polymer stages. The second of said applications also describes soaking the first stage substrate with the second stage monomer(s) to cause an "entangling" thereof with the silicone prior to subsequently polymerizing the second stage.
The polyorganosiloxane-based rubber utilized in the foregoing references tends to have poor mechanical properties, poor appearance properties, a tendency to cause delamination in final molded products and generally offers poor impact strength performance in typical thermoplastic applications.
Each of these disadvantages can be overcome by improving the elastomeric properties of the rubber. This objective can be accomplished by the practice of the present invention, where polyorganosiloxane rubber substrate is replaced by a co-homopolymerized network(s) of polyorganosiloxane/polyvinyl-based polymer(s) in a co-homopolymerization process. Polyorganosiloxane/polystyrene-based co-homopolymers are particularly preferred for use in the first stage of the graft polymers of the present invention.
None of the references disclose the in-situ co-homopolymerization of vinyl monomer in the presence of siloxanes in an emulsion system, as described hereinbelow. Furthermore, the present invention is also directed to the graft polymers provided by subsequent graft polymerization of vinyl-based monomers (e.g. polymethyl (meth)acrylate, polystyrene or styrene/acrylonitrile copolymer) in the presence of the polyorganosiloxane/ polyvinyl-based polymer product of the foregoing step.
It will be seen that this multi-stage graft polymer is usefully employed as an impact strength modifier for thermoplastic molding compositions while maintaining many unique features of silicone rubber. Compounded thermoplastic resins containing the graft polymer obtained from the process disclosed herein exhibit enhanced impact strength, particularly at low temperature, while maintaining other properties, such as weathering and thermal resistance.